1. Field of the Invention
This invention relates to a hydrogenative treatment process for refining petroleum distillation residues. More specifically, the invention is directed to such a process in which petroleum distillation residues are magnetically treated to remove iron contents therefrom prior to hydrogenative treatment.
2. Prior Art
As has been commonly practiced in the art of petroleum refining, residual oils resulting from atmospheric or vacuum distillation of a crude petroleum oil are subjected to cracking, desulfurization and other reactions upon passage through a fixed-bed hydrogenation reactor.
In most cases, such residual oils contain considerable proportions of particulate iron or iron compounds which emanate during the transport of a crude oil from a shipping tanker through storage tank and delivery pipe lines to a distillation plant. Such iron impurities tend to deposit on a catalyst bed or in between individual catalyst particles, resulting in a plugged up reactor or deteriorated catalyst. A plugged up reactor would often lead to objectionably increased pressure to a point where the plant operation has to be discontinued. Particulate iron impurities present in petroleum distillation residues are usually of the order of 0.1-20 microns in size, too small to be removed by relatively large mesh filters commonly used at petroleum oil refineries.
There may be considered several alternatives for removing the iron impurities from petroleum residual oils to be treated. One would be to use a fine mesh filter such as a filter cloth or paper, but such filters entail large pressure loss, are easy to get clogged and very tedious to replace, and hence not suitable for application in petroleum refining where massive crude oil is handled. Another alternative would be to use a centrifugal separator, but this is practically infeasible in view of its structural and operational limitations.
In U.S. Pat. No. 4,836,914 and Japanese Laid-Open Patent Publication No. 62-54790 there is disclosed the use of a high gradient magnetic separator for iron removal. While the disclosed device can be successfully operated for certain initial periods of time, it has been found that the efficiency of removal of iron contents in a petroleum residual oil declines progressively with time chiefly due to fouling of the ferromagnetic filler by those iron particles which are continually deposited thereon in the magnetic field.